In recent years, a bio-diesel fuel made from biological oils attracts attentions as one of countermeasures against global warming. Utilized are various fats such as colza oil in Europe, Pistacia chinensis and the like in China, soybean oil in North, Central and South America and oil palm, coconut palm and Jatropha curcas in Southeast Asia.
There is concern that direct use of such fats, which have a feature of high viscosity among others, as they are as fuel for a diesel automobile would cause failure of an engine due to adhesion of deposits on a fuel pump. Thus, used is fatty acid methyl ester (abbreviated as FAME using initials) or the like with physical properties similar to those of diesel oil and obtained through conversion of the raw fats by removing glycerin therefrom in a chemical treatment such as methyl esterification.
More specifically, the fats are added with methanol and a catalyst to cause an ester exchange reaction. The resultant material is added with an acid to perform neutralization and is separated into FAME and glycerin. The separated FAME is washed with water for removal of the catalyst and is distilled for removal of methanol to obtain a biodiesel fuel.
This kind of biodiesel fuel has a feature of being easily oxidized in the presence of oxygen. When a vehicle with the biodiesel fuel in its fuel tank is left unused for a long time, the fuel in the tank may proceed to oxidization and deterioration to corrode the tank and/or produce polymers to cause clogging in fuel injection and lubricating systems, resulting in serious damages to the engine. Thus, countermeasures against oxidation of a biodiesel fuel in a fuel tank have been demanded.
There exists a conventional technique, for example, in the following Patent Literature 1 that nitrogen-rich air is supplied from a nitrogen gas cylinder to a fuel tank to lower a concentration of oxygen in the tank. In Patent Literature 1, air is separated into oxygen- and nitrogen-rich airs by an oxygen/nitrogen separator with an oxygen enrichment membrane and the nitrogen-rich air is accumulated in the nitrogen gas cylinder.